Ferroresonant microwave attenuator



Nov. 6, 1962 w. A. HUGHES 3,063,029

FERRORESONANT MICROWAVE ATTENUATOR Filed March 11,1954

2 Sheets-Sheet l Nov. 6, 1962 W. A. HUGHES FERRORESONANT MICROWAVE ATTENUATOR Filed March 11, 1954 2 Sheets-Sheet 2 United States atent 3,063,029 Patented Nov. 6, 1962 Filed Mar. 11, 1954, Ser. No. 415,475 i 12 Claims. or. 333-451) This invention relates to waveguides for microwave transmission, and more particularly to an improved microwave attenuator of a type in which the amount of attenuation is controlled electromagnetically.

Electromagnetic control of attenuation in a waveguide is achieved by inserting ferromagnetic material in the waveguide and applying an external magnetic field per pendicularly to the axis of the waveguide. The structure and behavior of some waveguide attenuators filled withferromagnetic material are discussed in an article entitled, Magnetically Controlled Waveguide Attenuators, by Theodore Miller,'Journal of Applied Physics, vol. 20, pp. 878-82 published by American Institute of Physics, Inc., 1949. A number of difliculties are inherent in attenuators of this type and have inhibited their use in practical devices. The difficulties include undesirable reflection of microwave energy in the waveguide, high insertion loss and abnormally large physical structures. In both mechanical and magnetic attenuators of the prior art, effective attenuation is restricted to a relatively narrow frequency band. It is sometimes desirable that, micro- Wave attenuators be effective over a broad spectrum of frequencies. Q

It is therefore an object of this invention to provide an electromagnetically controlled waveguide attenuator in which reflection of transmitted or driving energy 'is sub stantially negligible.

It is another object of this invention to provide a waveguide attenuator in which attenuation is electromag netically controlled, wherein insertion losses are minimized and attenuation is effective over a wider range of frequencies than has been possible .with prior art waveguide attenuators.

It is still another object of this invention to provide an attenuator which is remotely controllable.

It is another object of this invention to provide a waveguide attenuator structure of the electromagnetically con trolled type, in which the physical dimensions of the structure are a practical minimum.

It is still a further object of this invention to provide a miniaturized waveguide attenuator structure employing aminimum number of component parts of simple design, capable of being easily reproduced and assembled.

In accordance with a preferred embodiment of thisinvention, a pair of rectangular pole pieces are embedded in opposite walls of a waveguide with their pole faces disposed in opposition with respect to each other. These pole pieces are located on one side of the longitudinal center-line of the waveguide. A' rectangular shaped ferromagnetic dielectric element is positioned between the pole pieces. The pole pieces are magnetically coupled to an external core, which supports an exciting coil. When the coil is energized to establish a sufli'ciently strong field between the pole faces, substantially all microwave power inserted in the waveguide flows through the ferrite.-

When the ferromagnetic dielectric element is subjected to the external magnetic field, part of the microwave power is converted to heat energy which is dissipated in the walls of the waveguide. The degree of attenuation is directly related to the degree of such conversion. In

the device of the present invention, there is substantially no reflection of the microwave energy, insertion losses l Insertion losses due to eddy currents are substantiallyv are a minimum, and substantially unity ratio of voltage standing waves is realized over a relatively wide range of microwave frequencies.

The novel features which are believed to be character istic of the invention, both-as'to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the folowing description considered in connection with the accompanying drawings in which several embodiments of the'invention are illustrated by way of example. It is to be understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention. Referring to the drawings, which are made a part of this specification,

FIG. 1 is a perspective view, including a cut-away portion, of a waveguide attenuator in accordance with this invention;

FIG.'2 is a bottom view of the device shown in FIG. 1; FlG. 3 is a view taken along the line 33 of FIG. 1; FIG. 4. is a sectional view taken along line 4-4 of FIG. 1;

FIG. 5 is an end elevation view of the device of FIG. 1; FIG. 6 is an exploded view of certain portions of the device of FIG. 1; i Y

FIG. .7 isa fragmentary perspective view of a portion of the device of FIG. 1;

\ FIG. 8 is a curve showing attenuation characteristics of the device of FIG. 1, for the purpose of explanation;

FIGS. 9 and 10 illustrate different modifications of a portion of the device of FIG. 1 to obtain distinct results, further in accordancewith this invention; and v H FIG. 11 is a block diagram of a servo system to illus-. trate a practical use of the Referring to the drawings, in which like reference characters indicate like parts throughout, and more particularly to FIGS. 1-7, a rectangular waveguide is formed essentially of two pieces, a bottom member 20 and top member .22. Bottom member 20 has an inner surface of step formation and preferably is solid metal in which steps are milled. Top member 22 has a broad wall 24 and downwardly depending side portions 26, 26' which are cut to match the steps of bottom member 20. The broad walls or surfaces of the waveguide thus are the broad wall 24 of top member 22 and the surfaces of the steps of bottom member 20. The side walls of the waveguide are formed by the side portions 26, 26 of top member 22. The-members 20, 22 can be readily assembled and secured together, for example, by silver-soldering.

negligible by this construction, as will be made moreevident hereafter.

A rectangular slot 28 extends through the wall 24- of top member 22 (see FIG. 6), and is located adjacent to one side of the waveguide. A portion of bottom member 26 is removed to provide a thin section 30 directly opposite slot- 28. A rectangular slot 32 to match slot 28 is provided in this thin section 30. A pole piece 40 is fixed within slot 28, and its pole face registers flush with the inner surface of the broad wall 24. Similarly, a pole permeable cup or shield 54 is piece 42 is fitted in slot 32, and its pole face is flush with the inner surface of the portion 30.

Pole piece 40 forms the shoe of a magnetizable core 50 and supports a coil 52 which is placed over the core. A positioned over coil 52, and external leads 56 are connected'through shield 54 to the ends of coil 52. Shield 54 is fixed to core 50, screw 57. A permeable plate 55 is aflixedto the open end of shield 54 to provide a complete magneticcircuit.

Flanges 58, 59 fixed to the opposite ends of the waveattenuator of this invention,

guide provide conventional means for connecting the ends of the waveguide to other waveguide apparatus.

When the waveguide is assembled as shown, the steps previously mentioned constitute a conventional microwave impedance transformer. For the purposes of this invention, such a transformer is of the type which has a broadband impedance match. This is convention-ally achieved with an odd number of steps, succesive steps being spaced a quarter of a wavelength apart. In the embodiment shown here, referring to FIG. 6, three steps 60, 61, 62 lead up from the bottom of the waveguide at its input end to the center section 64 of member 20. Center section 64 includes the thin section 30. The impedance transformation is the ratio of the height of the waveguide at its input end to the height of the waveguide at center section 64.

Although the transformer above described has a broadband impedance match, there may be an undesirable amount of inductive susceptance in the plane of the highest step 62. The inductive susceptance tends to cause impedance mismatch at this point and consequent reflection of energy. Steps 62 and -62"are'provided respectively with thin cells 63 and 63', each of which project above the surface of center section 64, and this is known in the art as a capacitive iris. This iris provides sufficient additional capacitance to cancel the inductive susceptance and provide effective impedance match over the desired band. The transformer above described serves to provide a minimum height of waveguide at center section 64. Accordingly, a relatively small amount of power is required to establish a desired magnetic field between the pole pieces 40 and 42. This is a factor of considerable importance, in that the saving in material and space to obtain the necessary external magnetic field strength is quite large compared with the requirements for waveguide attenuator structures of similar type hitherto known. In addition, the reduction in waveguide height is effective to minimize the possibility of the existence of modes in the vertical direction; the importance of this aspect will be made more evident hereafter.

If the output of the attenuator waveguide is to be applied to a waveguide of the same height as the input waveguide, a second transformer similar to that above described includes successive descending step 62, 61 and 60 leading from center section 64 to the bottom of the waveguide at its output end. The impedance transformation is the reverse of that for the transformer previously described.

Fixed between the pole faces of pole pieces 40, 42 is an elongated rectangular shaped attenuator vane, or element, 70. Element 70 is a ferronmagnetic dielectric element, the dielectric characteristics of which determine the insertion loss. Such a ferromagnetic dielectric, or ferrite, may be composed of a ferric-oxide combined with a bivalent meta1, as XFe O and XFe O where X represents the bivalent metal. Magnesium and manganese represent a suitable bivalent metal.

The length of element 70 is sufiicientlyless than the length of center section 64 to permit highly reactive fields (i.e., higher order modes), which may exist in the center section 64 on account of the step transitions to be suppressedby the waveguide itself in theregions between the endsof the element 70 and the adjacent steps 62, 62'. The thickness of element 70 is less than the internal height of the waveguide at center section 64 to minimize any impedance changes which occur in teh region of the ferrite.

The arangement of pole pieces 40, 42, and the element 70 locates the element 70 out ofthe region of high electric field intensity and in the region of-the circularly polarized alternating magnetic fields which form a part of the microwave power passing through the waveguide.

Referring to FIGS. 4 and 5, the portion of core 50 adjacent 'pole piece 40 tapers from its outer diameter to the bottom, of pole piece 40. In this manner, substantially all the magnetic flux is focused in pole piece 40 and directed through the element 70.

The operation of the structure above described will now be explained for the waveguide excited in the T E mode, In the absence of an externally applied magnetic field, the electric and magnetic fields are set up as usual, with the transverse electric field across the height of the waveguide and the alternating magnetic fields extending across the width and along the length of the waveguide. In the absence of a static magnetic field, element 70, because of its location and size as previously explained has substantially negligible effect on the power propagated.

The electrons in the element 70 have an average magnetic moment which is random in the absence of the external field. When coil 52 is energized to establish the transverse external field, the average magnetic moment tends to line up with the external field established. In addition, the alternating magnetic fields in the ferrite cause this average magnetic moment to precess. The precessing of the average magnetic moment is attended by the production in the ferrite of magnetic fields which tend to reinforce the alternating magnetic fields. This results in a shift of the microwave power over to the region of the ferrite. The precession and shift of microwave power depend upon the strength of the external field. Further, the microwave power is absorbed in attendant precessional damping losses, rnanitested as heat dissipated through the walls of the waveguide.

The degree of attenuation of the microwave power depends upon the extent to which it is absorbed, and maximum absorption occurs at the value of external field strength at which precessional, or gyromagnetic, resonance exists. Precessional resonance occurs when the frequency of precession of the average magnetic moment is the same as the frequency of the alternating magnetic fields, i.e., the same as the so-called driving frequency.

FIG. 8 illustrates the variation of attenuation with external field strength. A value H of the external field strength is reached before precession of the average magnetic moment is sufiicient to realize any appreciable absorption and attenuation. Attenuation then increases with the field strength until, at a field strength H,, the attenuation is maximum; this is the point of precessional resonance. For greater field strengths, the attenuation falls off sharply, as indicated. Accordingly, depending upon the attenuation desired, the external field strength should not exceed H,.

In one practical embodiment of a waveguide attenuator of the type above described, the waveguide was 0.4-" high at its ends. A transformer of the type described was used in which the successive steps changed the height of the waveguide in the relation 1:2:1 to make the height of the waveguide 0.1- at center section 64. Further, impedance match covered 20% of the X-band. Other specifications were: The impedance transformation was thus 4:1.

Body members 20, 22 and flanges 58, 59: brass.

, Core 50: soft iron, 0.375 in. diameter.

Shield 54: soft Pole pieces 40, 42: soft iron, 0.800 in. by 0.150 in.

Coil 52: 27,000 turns, number 38 copper wire.

7 iron, 2-in. diameter, 1.60 in. high.

Element 70: Ferrite composed of 51% iron, 11% magnesium, 4.80% manganese, 33.20% oxygen, 0.825 in. x 0.25 in. x 0.050 in.

An arrangement for insuring X-band. It has not been possible heretofore to maintain a VSWR less than 2.0 over more than 8% of the X-band.

FIG. 9 illustrates a modification of the magnetic circuit previously described. Referring to FIG. 9, the bottom pole piece 42' has a circular base from which there is a transition to a rectangular parallelepiped. The rectangular portion is positioned in the waveguide opposite pole piece 40, in the maner previously described for pole piece 42. Positioned under the base of pole piece 42 is a short, cylindrical permanent magnet 75 to be alfixed to the shield 54. The magnet 75 provides a fixed flux density between pole pieces 40, 42'. The preferred value of this fixed field is that which the coil 52 alone would have to provide before attenuation would be effective, i.e., H in FIG. 8. -The size of coil 52 and the power required to establish the desired flux density is thus less than would be required without the permanent magnet 75.

As pointed out previously, the waveguide attenuator arrangement of LFIG. 1 provides maximum attenuation when the external field strength is sufi icient to cause precession of the averagemagnetic moment at the driving frequency. Thus, if the driving frequency varies with the external field strength remaining the same, the attenuation at the different frequency will not be exactly the same because precessional resonance is not as pronounced. precessional resonance over a range of frequencies for one value of the external field strength is obtained by shaping at least one of the pole pieces to effect a variation of flux density across the width of the ferrite element 70, as illustrated in FIG. 10.

Referring to 'FIG. 10, a modified upper pole piece 40 has an irregularly configured pole face. The thickness of the portion of the pole face registering with the inner surface of wall 24 is less than the width of the pole piece 44). This is a depending portion 40', illustrated as an extension of the side of the pole piece 40' nearest the axis of the waveguide. The greatest concentration of the external field is between portion 40" and bottom pole piece 42. The field varies between pole piece 42 and the more widely spaced parts of the pole face. Lines drawn from pole piece 40' indicate generally the flux paths set up. This construction makes use of the shifting with driving frequency of the longitudinal plane containing the circularly polarized alternating magnetic fields. Al-' though the element 70 is located in the region of these fields, the plane of circular polarization obviously will move across the width of the element, from its inner edge to its outer edge, as the frequency decreases. The configuration of pole piece 50 insures a variation in flux density across the width of element 70, so that the plane of circular polarization at any point across element 7t) is located in the flux path which is of the proper strength to maintain precessional resonance. In this manner, precessional resonance is maintained in spite of variations in the driving frequency, and hence maximum attenuation is maintained.

It is to be understood that it is not required to use external field strengths which will give precessional resonance. The attenuator of this invention can be set to provide less than maximum attenuation, as desired.

FIG. 11 illustrates a servo system employing the attenuator of this invention. Included in the system is a klystron oscillator 90 having a waveguide 91 connected to its output, the attenuator 92 of this invention connected between waveguide 91 and an output waveguide 93, a directional coupler and detector 94 connected to waveguide 93, an amplifier 95 and control amplifier 96 coupled between coupler '94 and attenuator 92. Control amplifier 96 supplies current to the attenuator control coil to control the external magnetic field. Voltages in waveguide 93 are applied through coupler and detector 94, amplified, and fed through control amplifier 96 to the attenuator control coil. For voltages tending to exceed a predetermined value, the external magnetic field automatically increases to maintain the output from wave guide 93 constant. Similarly, output voltages tending to fall below the desired value, give rise to a decrease in the external magnetic field strength to decrease the attenuation by a suflicient amount to maintain a constant output. Thus an output utilization device 98, which requires a constant input voltage, can be connected to waveguide 93.

What is claimed is:

1. In a microwave component wherein at least one ferrite vane is mounted entirely in a rectangular waveguide and is subjected to an externally applied static transverse magnetic field, a pair of conductive ferromagnetic pole pieces built into opposite broad sides of the Waveguide for mounting the ferrite vane to channel the magnetic field through the ferrite Vane while maintaining substantially unimpaired the conductivity of said broad sides of the waveguide.

2. In a microwave component which utilizes the ferroresonant properties of a ferrite vane mounted entirely within a waveguide and positioned in a static magnetic field for altering a microwave signal, the combination comprising: a section of waveguide; at least one conductive ferromagnetic pole piece built into a side wall of said waveguide, an end of said pole piece lying essentially flush with the inner surface of said waveguide wall; a fer rite vane mounted within said waveguide on said pole piece; and a static magnetic field source having first and second poles and positioned around at least a portion of said waveguide for producing a static magnetic field in the interior of said waveguide, said first pole of said source engaging said pole piece whereby said field is channeled through said ferrite vane.

3. The combination defined in claim 2 wherein said waveguide is rectangular in cross section and which further includes a second pole piece built into the wall of said waveguide opposite said one pole piece and surfacing both within and without said waveguide, said second pole of said source engaging said second pole piece.

4. The combination defined in claim 3 wherein said static magnetic field [sic.] source includes a permanent magnet.

5. The combination defined in claim 2 wherein said static magnetic field source comprises an electromagnet, whereby said microwave signals are altered upon ener gization of said electromagnet.

6. An absorption type microwave component comprising: a rectangular waveguide; first and second ferromagnetic pole pieces built into opposite walls of said Waveguide, said pole pieces being parallel to each other and to the longitudinal axis of said wave guide and surfacing both within and without said waveguide, said pole pieces substantially preserving the electrical conductivity of the walls of which they [sic.] form a part; at least one ferrite vane mounted within said waveguide on at least one of said pole pieces, the electrical conductivity of said vane being relatively low compared to that of said pole pieces;

and a magnetic field source positioned around at least a portion of said waveguide and having first and second poles, said first and second poles engaging said first and second pole pieces, respectively, for producing a magnetic field through said ferrite vane.

7. In a microwave component which utilizes the ferroresonant properties of a ferrite vane mounted entirely within a waveguide and positioned in a static magnetic field for altering a microwave signal, the combination comprising: a section of rectangular waveguide; at least one conductive ferromagnetic pole piece built into the wall of said waveguide and in electrical contact therewith; said pole piece surfacing both within and without said waveguide; a ferrite vane mounted within said waveguide on said pole piece; and a magnet positioned adjacent the exterior of said waveguide and having first and second poles, said first pole of said magnet engaging said pole piece whereby said ferrite vane is included in a low reluctance path with said magnet.

8. An absorption type microwave component comprising: a rectangular waveguide; first and second ferromagnetic pole pieces built into opposite walls of said waveguide and being composed of material whose conductivity is of the same order of magnitude as that of said waveguide, said pole pieces being parallel to each other and to the longitudinal axis of said waveguide and surfacing both within and without said waveguide; at least one ferrite vane mounted within said waveguide on at least one of said pole pieces; and a static magnetic field source positioned adjacent said waveguide and having first and second poles engaging said first and second pole pieces, respectively, for producing a magentic field through said ferrite vane.

9. A waveguide attenuator comprising a length of rectangular waveguide having an input end of predetermined height, a broad band impedance transformer in said waveguide extended from said input end increasing in height in discrete steps to provide a waveguide section having a minimum height proportional to said predetermined height, at least one pole piece of ferromagnetic material extending into a wall of said waveguide section, the end of said pole piece lying essentially flush with the inner surface of said wall, a thin elongated attenuation strip of ferrite disposed in said section and contacting said pole piece with the longitudinal axis parallel to the sides of said section and a broad surface parallel to a broad wall of said section at least substantially between the center line and one side of such broad wall, said strip having a thickness substantially less than said minimum height and a Width less than half the width of said waveguide, and means for establishing a static magnetic field through said pole piece and said ferrite transverse to said broad surface and along the length thereof of a value producing gyromagnetic resonance at the frequency of microwave energy propagated through said waveguide whereby resonance absorption of energy by said ferrite occurs.

10. A microwave attenuator comprising a length of rectangular waveguide having an input end of predetermined height between broad walls for propagating microwave energy in a plane polarized magnetic field mode parallel to the broad walls, impedance transformer structure included in said waveguide and extended the width of said waveguide for a predetermined length from said input end for providing a waveguide section having a height between broad walls proportionately less than said predetermined height whereby an impedance proportional to the relative heights at the respective ends of said structure is provided with an impedance match minimizing reflections, at least one pole piece of ferromagnetic material extending into a wll of said waveguide section, the end of said pole piece lying essentially flush with the inner surface of said wall, an elongated segment of ferrite material mounted in said section entirely on said pole piece with its longitudinal axis parallel to the narrow walls of said section between the longitudinal center line and one narrow wall to include a plane of circular polarization of said magnetic field mode, means for establishing a static magnetic field through said pole piece and said segment parallel to the narrow walls of said section of a value to produce gyromagnetic resonance at broad surface of a value said plane of circular polarization at the frequency of energy propagated through said waveguide whereby energy is absorbed by resonance absorption in said ferrite and resultant heat dissipated through adjacent walls of said waveguide.

11. In a length of rectangular waveguide for propagating microwave energy in the TE mode having a magnetic field parallel to the broad walls and including impedance matching transformer structure proportionately decreasing the height of the narrow walls to provide a ssection of reduced height with minimum reflection, at least one pole piece of ferromagnetic material extending into a broad wall of said section, the end of said pole piece lying essentially flush with inner surface of said broad wall, an attenuator structure comprising an elongated ferrite element mounted against said pole piece and in said section between the longitudinal center line and one side of said section to include a plane of circular polarization of said magnetic field, said element having a thickness less than half said reduced height and a width less than half the dimension of the broad walls of said section, and means for establishing a static magnetic field through said pole piece and said element parallel to the narrow walls of a value producing gyromagnetic resonance at the frequency of said energy along said plane of circular polarization whereby energy is absorbed in said ferrite by resonance absorption and resultant heat is dissipated by adjacent walls of'said waveguide.

12. A waveguide attenuator comprising a length of rectangular waveguide having impedance transformer means at input and output ends and a central section of reduced internal height, at least one pole piece of ferromagnetic material extending into a broad wall of said section, the end of said pole piece lying essentially flush with the inner surface of said broad wall, a thin elongated ferrite strip mounted within said center section in contact with said pole piece and with its broad surface parallel to said broad wall of said center section and disposed at least substantially at one side of the longitudinal center line of said broad wall, said ferrite strip having a thickness substantially less than said reduced internal height whereby insertion losses are minimized, and means for establishing a static magnetic field through said pole piece and said ferrite strip transverse to said to produce gyromagnetic resonance at the frequency of microwave energy propagated through said waveguide whereby said energy is attenuated by resonance absorption in said ferrite.

References Cited in the file of this patent UNITED STATES PATENTS 2,457,601 Ring Dec. 28, 1948 2,531,437 Johnson et al Nov. 28, 1950 2,629,079 Miller et al. Feb. 17, 1953 2,646,551 Roberts July 21, 1953 2,745,069 Hewitt May 8, 1956 2,844,789 Allen July 22, 1958 2,849,684 Miller 'Aug. 26, 1958 OTHER REFERENCES Journal of Applied Physics, vol. 24, No. 6, June 1953, pages 816-817. 

1. IN A MICROWAVE COMPONENT WHEREIN AT LEAST ONE FERRITE VANE IS MOUNTED ENTIRELY IN A RECTANGULAR WAVEGUIDE AND IS SUBJECTED TO AN EXTERNALLY APPLIED STATIC TRANSVERSE MAGNETIC FIELD, A PAIR IF CONDUCTIVE FERROMAGNETIC POLE PIECES BUILT INTO OPPOSITE BROAD SIDES OF THE WAVEGUIDE FOR MOUNTING THE FERRITE VANE TO CHANNEL THE MAGNETIC FIELD THROUGH THE FERRITE VANE WHILE MAINTAINING SUBSTANTIALLY UNIMPAIRED THE CONDUCTIVELY OF SAID BROAD SIDES OF THE WAVEGUIDE. 